Moed Katan, Chapter One, Mishnah Nine
This mishnah deals with setting up various instruments needed for the preparation of food.
1) They may set up an oven, stove or a millstone during the festival.
2) Rabbi Judah says: they may not roughen millstones for the first time.
Section one: Setting up an oven, stove or a mill involved assembling the separate parts and connecting them with plaster. The mishnah allows this because it is not a lot of work and it is necessary for the festival.
Section two: For the millstones to grind well, their face was had to be roughened by putting grooves and ridges on it. While Rabbi Judah agrees that one can set up the millstone, he doesnt allow it to be roughened for this is strenuous work. The roughening also could have been done before the festival.